1. Field of the Invention
The present invention relates to a method and to an apparatus for making structural reinforcement preforms for resin transfer molding (RTM) and reaction injection molding (SRIM) processes for structural composites, and is further particularly concerned with the handling of reinforcement webs used in the process and in attaching reinforcement members and the like as a part or parts of the preforms.
2. Description of the Prior Art
As set forth in our aforementioned patent application Ser. No. 446,859, filed Dec. 6, 1989, in making preforms according to the directed fiber process, it has heretofore been the practice to spray chopped fibers with a binder resin onto a form that has air pulled therethrough to locate and hold the fibers in place. The form with the fibers and the binder resin thereon is then moved into a hot air plenum chamber, dried and/or cured to set the binder resin. In addition, a great deal of processing space is required for drying, curing and cooling the preforms.
In making thermoformed preforms, it is heretofore been the practice to use a continuous strand fiber mat that has been previously coated by the fiber manufacturer with a thermoplastic binder. The thermoformable mat is supplied in a roll form whereby it is unrolled into flat sheets of varied layer thicknesses and clamped into a holding frame at the edges. The frame network is then positioned in an oven chamber containing radiant heaters which slowly heat the reinforcement mat and the thermoplastic binder from both sides Upon heating, the thermoplastic binder softens and, while soft, the frame network is quickly transferred into a cold mold. The mold closes via a press forcing the reinforcement mat into the desired shape of a part. Upon cooling, the thermoplastic binder stiffens and thus holds the thermoformable mat in its new shape.
As pointed out in our aforementioned patent application Ser. No. 446,859, these processes are slow, require a great deal of space and a large amount of energy.
As also pointed out, in conventional RIM/SRIM process applications for structural components, fiber layer thickness across the entire preform is increased to meet the strength requirements of one area, which results in unnecessary use of material in other areas and increases thickness and weight. Furthermore, neither the directed fiber process nor the thermoformable mat process allows a designer to add ribs or closed sections to maximize design properties.
In our aforementioned application, therefore, we proposed a new system which eliminates the necessity for large rooms and constantly operating ovens, cooled presses and the like and permits design flexibility with respect to the provision of reinforcement ribs, closed sections, and reinforcement and/or attachment members while at the same time saving on energy and materials.
Our new process, as disclosed in application Ser. No. 446,859, utilizes specifically-developed binders along with directed energy systems for rigidizing the composite forms and attaching structural components to the preforms and is entirely compatible with RTM and SRIM resin systems, i.e. polyesters, vinyl esters, urethanes, epoxies, phenolics and acrylics. The process is designed to be fully automated and to enable specific distribution and placement of numerous type of reinforcements, where necessary, for the required structural properties of a preform. There is a complete freedom of design inherent in the process and allows for the most desirable reinforcement type and/or structures including closed structural shapes and varied wall sections to meet design criteria.
In the process disclosed in the aforementioned application, mats of reinforcement material are cut into a desired shape as a two-dimensional planar development of a desired preform. The cut mats are then coated with a binder which is responsive to electromagnetic energy, either microwave radiation or ultraviolet radiation, and the cut mats are placed in a three-dimensional mold and pressed to replicate the desired shape of the preform.
While in the mold, the coated and shaped mats are subjected to the appropriate electromagnetic radiation, either microwave or ultraviolet radiation, to cure the binder resin and provide rigidity in a matter of seconds, rather than minutes or hours as with the heat-curable processes. At this point, the preform is a finished product for use in a further molding operation (RTM, SRIM) or may be viewed as a carrier preform for the attachment of structural reinforcement members and the like before being used in a further molding operation (RTM, SRIM).
As a carrier preform, the rigid three-dimensional preform is removed from the mold to a station where a designated area or areas of the preform or of a subassembly (reinforcement rib) are provided with a further coating of an electromagnetic energy curable binder resin, the subassembly (reinforcement member or the like) is moved into intimate contact with the preform at the coated area or areas and the appropriate electromagnetic radiation is applied to energetically stitch (cure the binder) the subassembly to the carrier preform. When the final attachment has been made by such energetic stitching, the preform is a finished product in itself ready for use as a structural reinforcement preform as a part of a further molding process for making a structural composite.
As a structural reinforcement preform the structure is hollow as the walls are permeable to the pressure-applied material during RTM/SRIM processing so that any pocket or chamber could fill with the plastic molding material causing waste, increased weight and longer curing time. Therefore, a core may be inserted in any such pocket or chamber to prevent or at least minimize such an event. The core may be held in place by a subassembly (cover) energetically stitched to the preform.
We have determined that the entire process may be improved, particularly with respect to the handling of the reinforcement material prior to bonding in that a plurality of layers of reinforcement material must be individually cut into the desired shape and individually stacked in registry in the mold. With such layers tacked together prior to cutting (termed energetic basting), so that the cut layers are essentially a single element, handling is simplified in that registration of the layers is then inherent.
We have also determined that the binder can be applied and the layers tacked together, prior to cutting, so that there is no necessity of later applying binder resin to individual layers after cutting.
We have also determined that the layers of reinforcement material may be drawn as webs from roll goods, the binder resin applied and the webs superposed and tacked together (energetically basted) at spaced local zones prior to cutting by selective curing of such zones with the remaining uncured binder resin available for later curing and rigidizing a shaped element in the mold.